AbstractAccording to their degree of morphological specialization, the cell populations of the brain stem may be classified into three groups: isodendritic, allodendritic and idiodendritic. The isodendritic neurons, or generalized neurons, are the most frequently encountered. If one discards those isodendritic centers that by common definition are sensory or motor, an isodendritic core is left which displays very little histological variation throughout the whole extent of the brain stem. This core corresponds, with certain restrictions, to the regions that are usually regarded as reticular formation. It constitutes a continuum of overlapping dendritic fields that extends from the spinal cord to the diencephalon. In view of the similarities that exist between the histology of the isodendritic core and the relatively disorganized nervous system of the lower vertebrates, it is postulated that it represents a pool of pluripotential neurons which in the course of phylogeny have remained relatively undifferentiated and in charge of processing afferent signals of very heterogeneous origin. By contrast, the allodendritic and idiodendritic centers can be regarded as relatively specialized centers from the point of view of their dendritic morphology, connections and functions. Attention is paid to the fact that the diffuse characteristics of the isodendritic core do not necessarily entail ill‐defined physiological properties.